1. Field Of The Invention
The present invention relates to gas discharge lasers, and in particular to excimer lasers, and more particularly to an electrode arrangement for such gas discharge lasers.
2. Background Of The Invention
Excimer lasers provide high intensity laser radiation in the ultraviolet spectral range. This makes them important tools especially for medical and surgical applications as well as for other industrial applications.
Excimer lasers are gas discharge lasers that use a rare gas such as argon and a halide gas such as fluor (for example ArF excimer laser) or a gas containing a halide (for example F2) as the laser gas.
Generally, in an excimer laser. a gas mixture containing the active component and other gases is steadily provided to a discharge gap between a pair of elongated electrodes inside the laser tube by means of a fan or the like. A high voltage applied between the two electrodes causes a gas discharge in said discharge gap, whereby, from the active component of the gas, short-lived excited-state molecules are generated, whose disassociation generates ultraviolet radiation constituting the laser radiation. To increase the homogeneity of the gas discharge, in present excimer lasers a pre-ionization of the laser gas by pre-ionizers is used. As the used laser gas needs to regenerate before it can be reused, excimer lasers are generally operated in a pulsed operation mode, wherein the laser gas in the discharge gap is being steadily replaced by fresh or regenerated laser gas provided by the fan.
The discharge electrodes of an excimer laser are usually located inside the laser tube.
The housing of an excimer laser generally consists of a metal tube having openings in a cylindrical wall on the upper side thereof. An insulating plate covers the open upper side. The metal tube and one of the discharge electrodes are grounded. A high voltage is applied to the second discharge electrode via a HV duct extending through the insulating plate.
One main problem of excimer lasers, which is still not satisfactorily solved, is the contamination of the laser gas due to the corrosive effect of the active components of the laser gas on many insulating materials which are widely used as insulators, especially on materials containing carbon molecular structures, such as many plastic materials, for example TEFLON(copyright). Due to this contamination the lifetime of the laser gas is reduced, which makes a frequent exchange of the laser gas necessary. To overcome this problem, U.S. Pat. No. 4,891,818 utilizes high-purity aluminum oxide (Al2O3) as insulator, on which the corrosive effect of the active components of the laser gas is by far reduced as compared to plastic materials.
Another, even more corrosion-resistive, material that can be used as insulators are fluorides.
However, even though corrosion-resistive materials are used, corrosion cannot be avoided, in particular in those that are subjected to the very aggressive laser light. However, the production of laser light cannot be avoided, as this is, of course, the purpose of the laser. To date, no attempt or suggestion has been made regarding how to protect the insulators from the enhanced corrosion resulting from exposure to the laser light.
A need therefore exist for a mechanism to protect insulators from the aggressive corrosion resulting from exposure to the laser light. In addition, a need exist for a mechanism to protect the insulators from the light generated by the gas discharge and from the pre-ionizers, as such light also results in enhanced corrosion of the insulators. Such a mechanism would not only extend the lifetime of the insulators, but also, and perhaps more importantly, significantly lower the dust in the gas mixture within the laser tube.
Directly related to the above problem is the problem, that exchanging of the gas and maintenance works are expensive and time-consuming. Moreover, they are hazardous activities, as the laser gases for excimer lasers are, besides their corrosive nature, highly toxic.
The present invention may be used in conjunction with the inventions described in the patent applications identified below and which are being filed simultaneously with the present application:
All of the foregoing applications are incorporated by reference as if fully set forth herein.
One object of the present invention is to provide an electrode arrangement for a gas laser, and in particular for an excimer laser, that minimizes contamination of the laser gas and thus increases the lifetime of the laser gas.
Another object of the present invention is to provide an electrode arrangement for a gas laser and in particular for an excimer laser that is easy to handle and yet powerful.
The above and further objects of the invention are achieved by an electrode arrangement for a gas laser comprising an elongated high voltage electrode, an elongated ground electrode disposed adjacent to the high voltage electrode, a gas discharge gap between the two electrodes in which the gas discharge for the laser is generated, an insulator element, a high voltage conductor extending through the insulator element and having a first end connected to the high voltage electrode, and a shadow plate interposed between the gas discharge gap and the insulator element for shielding the insulator element against laser radiation irradiated from the gas discharge gap, as well as from gas discharge radiation and pre-ionization radiation.
The shadow plate is preferably made of a high purity metal, such as aluminum. Alternatively, it can be made of an insulator material, such as a heat resistant plastic material, such as TEFLON(copyright), or a ceramic material, which is then covered by a metal, preferably of high purity. According to still a further alternative the shadow plate can be made of any other material that is resistant against the laser gas and the laser radiation, gas discharge radiation, and pre-ionizer radiation.
The shadow plate can be arranged at any location between the gas discharge gap and the insulator element. For example, it can be located directly above the discharge gap on the high voltage electrode, or it can be located on the high voltage conductor. Preferably, the shadow plate is interposed between the high voltage electrode and the high voltage conductor.
The insulator may comprise an elongated shape and extend continuously substantially over the entire length of the laser tube and thus form a prior art electrode plate, which serves as an upper cover for the laser tube, and through which the high voltage electrode extends.
Preferably, however, the insulator element comprises a more compact form. Thus, an electrode arrangement according to a preferred embodiment according to the present invention comprises an elongated electrode plate made of an electrically conductive material and having a plurality of spaced-apart holes therein and a plurality of waveguide-like coaxial high voltage ducts, wherein each duct extends through one of the holes in the electrode plate. Each high voltage duct comprises a central conductive core having a first end and an insulator element, preferably made of a ceramic material. The insulator element is disposed around the core, preferably in a concentrical manner, to electrically insulate the core and electrode plate from each other. The electrode arrangement according to the preferred embodiment further comprises an elongated high voltage electrode electrically connected to the first end of the cores of the ducts, an elongated ground electrode positioned to oppose the high voltage electrode and form a gas discharge gap therebetween, the ground electrode being electrically connected to the electrode plate, and a shadow plate interposed between the gas discharge gap and the insulator element of each of the ducts.
As the electrode plate is grounded, an insulator is required between the conductive cores of the HV ducts and the electrode plate. The number of high voltage ducts (and holes) depends on the size of the laser, in particular on the length of the electrodes. For example, for a typical excimer laser, three high voltage ducts should be used. For a larger laser with longer electrodes more than three ducts should be provided. For smaller lasers with shorter electrodes only one or two ducts may be provided.
For practical reasons, the coaxial ducts comprise a round cross-section so that commercially available parts can be used. In principle, however, the ducts can comprise any cross-sectional shape, such as square or rectangular or oblong or anything else.
The shadow plates can be flat. Preferably, however, the edges of the shadow plate are bent towards the insulator elements. The edges of the shadow plate are preferably bent towards the insulator element for ensuring more efficient shading of the insulator. Furthermore, the shadow plate desirably comprises a flow-guiding shape to help guide the laser gas mixture, which typically comprises the laser gas and a buffer gas into the gas discharge gap.
The preferred discharge unit further comprises a pair of standard corona pre-ionizers, that is a pair of elongated cylindrical pre-ionizers with a conductive core and a surrounding tube-shaped insulator. The pre-ionizers extend substantially parallel along opposite sides of the electrode. The insulator of the pre-ionizers can be TEFLON(copyright), or any suitable insulator, but it is preferably a ceramic material, such as aluminum oxide. The insulator can also be a fluoride material. Alternatively, any other kind of known pre-ionizer can be used. The pre-ionizers are not necessary for the discharge unit to work. Indeed, excimer lasers were known before the invention of pre-ionizers. Pre-ionization, however makes the gas discharge between the high voltage electrode and the ground electrode more homogeneous and thus more reliable.
The overall construction of the laser can be such that first a laser tube is provided, and then the high voltage electrode, the ground electrode, the insulator element or elements, the high voltage conductor or conductors, the pre-ionizers and the shadow plate are mounted to the tube, one by one. It is preferred, however, that the electrode arrangement is a pre-mounted module-type discharge unit, wherein the electrodes, the shadow plate, the pre-ionizers and the high voltage ducts are pre-mounted independently of other laser elements. In this manner, the discharge unit can be mounted to the laser tube as a whole. This provides several advantages. One advantage is that the gas discharge gap between the high voltage electrode and the ground electrode can be adjusted before the discharge unit is mounted into the laser tube. This facilitates an accurate adjustment of the gas discharge gap. Furthermore the mounting of the laser can be done in a more efficient manner.
The laser gas can, in the case of an excimer laser, be any excimer laser gas, such as KrF, ArF, XeF, XeBr, HgBr, HgCl, XeCl, HCl, F2, Ar2 and the like or any laser gas in case of some other gas discharge laser.
Besides the laser gas, a buffer gas comprising a mixture of Helium, Neon and/or Argon is preferably provided in the tube.